Combustion processes that occur within internal diesel combustion engines create exhaust gases that contain nitric oxides, or NOx, an exhaust gas constituent that is the subject of certain governmental regulations.
One known process for reducing the nitric oxides content in engine exhaust is SCR. SCR involves chemical reaction, promoted by a suitable catalytic system, between nitric oxides present in the exhaust and a reductant that is introduced into the after-treatment system specifically as a reducing agent. It is known to introduce reductant as an aqueous urea solution that is able to release ammonia by hydrolysis under suitable temperature conditions or due to the action of specific catalysts directly into the exhaust gas stream upstream of an SCR catalyst. The introduction of urea solution may be closed-loop controlled by a processor that analyzes relevant data, such as backpressure, NOx, temperature, and ammonia leakage collected from corresponding sensors, and causes dosing apparatus to meter the solution based on results of the analysis.
The urea solution may be introduced by any of various methods, such as by nebulization in which the liquid is mixed with pressurized air and the mixture is allowed to expand into the exhaust stream, or by injection using a specific injection device, or injector, to flow the liquid at a certain pressure through a nozzle or valve into the exhaust stream without the use of air. The latter method, unlike the former, doesn't seem to promote the unwanted precipitation of solids out of solution. Nebulizing systems also tend to be less cost-effective, and more functionally complex, than injection systems.
On the other hand, an airless process may not disperse the liquid within the exhaust stream as well as one that is air-assisted. To improve dispersion in an airless process, a static mixer may be employed upstream of the SCR catalyst but the extent of improvement may be limited. Furthermore, the inclusion of such a device makes a usually unwanted contribution to system backpressure. Some static mixers impart tangential velocity components to the exhaust with respect to the exhaust flow axis and those components tend to promote concentration around the outer margin of the flow stream which can propagate downstream even as far as the SCR catalyst.
Non-uniform dispersion and incomplete dissolving of solution within the exhaust flow stream are known to impair efficiency of chemical and catalytic processes, and consequently, should be avoided in a commercial product.